JP4441166B2

JP4441166B2 - Method for improving environmentally-assisted cracking resistance of steel structure products

Info

Publication number: JP4441166B2
Application number: JP2002318155A
Authority: JP
Inventors: 明博宮坂; 知徳冨永
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2002-10-31
Filing date: 2002-10-31
Publication date: 2010-03-31
Anticipated expiration: 2022-10-31
Also published as: CN100439519C; JP2004149880A; KR100664003B1; KR20050065664A; WO2004040022A1; AU2003280700A1; CN1708592A

Description

【０００１】
【発明の属する技術分野】
本発明は、化学プラントや石油精製装置などの各種の装置や配管、或いはガスパイプラインなどとして使用される鋼構造物或いは構造製品において、塩化物、アミン、硫化水素、硝酸塩、炭酸塩などの各種の腐食環境下における応力腐食割れ、及び水素脆化割れ（以下、これらを環境助長割れという）抵抗性を高める、環境助長割れ抵抗性向上方法に関する。
【０００２】
【従来の技術】
環境助長割れとして、低濃度塩化物水溶液中あるいは塩化物の濃縮希釈が繰り返される海岸近傍における構造物，配管などの応力腐食割れ、石油精製工業の水素化脱硫装置の反応塔、加熱炉の熱交換器などにおけるオーステナイト系ステンレス鋼溶接熱影響部のポリチオン酸による粒界応力腐食割れ、硝酸アンモニウム製造装置、油井管、熱風炉などにおける炭素鋼、低合金鋼の硝酸塩水溶液環境下での応力腐食割れ、天然ガス輸送パイプライン，高圧ガスタービンなどにおける炭素鋼の炭酸塩水溶液環境下の応力腐食割れ、肥料用アンモニアの貯蔵、運搬容器などにおける炭素鋼の溶接熱影響部の応力腐食割れ、化学プラントにおける炭素鋼のＣＯ−ＣＯ_２−Ｈ_２Ｏ環境下での応力腐食割れなどがあり、さらに、Ｎｉ合金においても、圧力容器などにおけるＮｉ合金の応力腐食割れがあり、その他ＣｕやＣｕ合金でも、湿潤大気、海水，淡水環境下で、また、Ａｌ合金では塩化物水溶液環境下で応力腐食割れが発生している。
【０００３】
また、水素の金属材料への侵入により発生する応力による割れ、すなわち、水素誘起割れがある。その発生は水素の侵入反応速度により大きく左右されるが、金属材料に負荷ないし残留する応力によって亀裂の進展が加速され、上記の応力腐食割れと同様の挙動となる。すなわち、これらの割れは、応力の存在下において金属構造製品の表面或いは表層の微小な割れが腐食環境によって進展助長されて生じるものである。
【０００４】
これらの割れは、材料、腐食環境、応力の存在の３つの条件が重畳されて発生するものであり、これらのうちのいずれかの条件を除くことにより割れの発生を抑制することができる。材料の面から、応力腐食割れを起こさない材料への変更、例えばオーステナイトステンレス鋼からフェライト系ステンレス鋼への変更、腐食環境の面から、カソード防食などの防食措置の採用、或いは、応力の面から、負荷応力を除去ないし低減するための熱処理（応力除去焼鈍）やピーニング処理など、各種の対策が従来から行なわれている。
【０００５】
しかしながら、材料の変更は、材料強度上の制約や腐食環境との組み合わせ上の制約などから、適切な選択ができる場合は限られている。また、応力除去焼鈍は、処理対象が大型の構造製品である場合は、熱処理するための大型の熱処理炉が必要となり、また固定構造物などでは熱処理自体も不可能である。また、材料によっては応力除去焼鈍熱処理による材質上の変化も制約となる。さらに、外部から加えられる応力に対してはこの熱処理は効果が無い。また、ショットピーニングによる残留応力の低減も有効であるが、熱処理と同様に、大型の金属構造製品については装置上の問題を抱えている。
【０００６】
このように、これら従来の対策は適用上の制約があり、またコストも嵩むことから、経済的かつ効率的な対策とはなり難かった。
【０００７】
なお、上述のピーニング処理によって溶接製品の溶接部の強度を向上させ、応力集中や微小応力欠陥を抑制する応力パターンを形成するための超音波衝撃エネルギーによる処理方法が提案されている（例えば特許文献１参照）。また、この衝撃処理により表面が平滑になることも知られている（例えば、非特許文献１参照。）。
【０００８】
しながら、これらの文献には、環境助長割れについては言及されていない。
【０００９】
【特許文献１】
米国特許第６，１７１，４１５号明細書
【非特許文献１】
ＳｕｒｆａｃｅＮａｎｏｃｒｙｓｔａｌｌｉｚａｔｉｏｎ（ＳＮＣ）ｏｆｍｅｔａｌｌｉｃＭａｔｅｒｉａｌｓ−ＰｒｅｓｅｎｔａｔｉｏｎｏｆｔｈｅＣｏｎｃｅｐｔｂｅｈｉｎｄａＮｅｗＡｐｐｒｏａｃｈ，Ｊｏｕｒｎａｌ．Ｓｃｉ．Ｔｅｃｈｎｏｌ．Ｖｏｌ．１５Ｎｏ．３，１９９９）
【００１０】
【発明が解決しようとする課題】
本発明は、上記の問題点を解決し、環境助長割れ抵抗性を向上させる方法を提供することを課題とする。
【００１１】
【課題を解決するための手段】
本発明は、上記の課題を解決するためになされてものであって、超音波で先端を振幅３０〜６０μｍ、周波数２５ｋＨｚ〜６０ｋＨｚ、出力０．２〜３ｋＷで振動させる工具を用いて鋼表面を打撃する超音波衝撃処理を、鋼構造製品の環境助長割れが問題となる箇所に施すことによって、その鋼表層の組織を改善し、よって耐環境助長割れ性に優れた鋼構造製品を得るものであり、また、さらには、この処理を行なう際の適切な前処理、ならびに処理後の検査を行ない、その効果を保証するものである。その要旨とするところは、以下のとおりである。
（１）鋼構造製品の応力が集中ないし残留するため環境助長割れが問題となる箇所に、先端部に１〜２ｍｍの曲率半径を有する先端ハンマーを有する超音波衝撃装置により、その先端を振幅３０μｍ以上６０μｍ以下、サイクル数４０ｋＨｚ以上６０ｋＨｚ以下、出力０．２〜３ｋＷ、又は、振幅５０μｍ以上６０μｍ以下、サイクル数２５ｋＨｚ以上４０ｋＨｚ未満、出力０．２〜３ｋＷのいずれかの条件で振動させ、処理対象部分を打撃する超音波衝撃処理を施し、表面から５０μｍ以上の厚さの表層の鋼組織を結晶粒の長軸の方向と表面とが、±１０°以下の角度であるとともに、結晶粒の長軸方向長さと短軸方向長さとの比が５以上であるに結晶粒からなる層状組織とすることを特徴とする鋼構造製品の環境助長割れ抵抗性向上方法。
（２）前記鋼構造製品の環境助長割れが問題となる箇所の鋼が、引張強度４９０Ｎ／ｍｍ^２級以上の鋼であることを特徴とする（１）に記載の鋼構造製品の環境助長割れ抵抗性向上方法。
（３）前記鋼構造製品の環境助長割れが問題となる箇所が、溶接ボンド部および／または溶接熱影響部を含むことを特徴とする（１）または（２）に記載の鋼構造製品の環境助長割れ抵抗性の向上方法。
（４）前記層状組織の結晶粒の長軸の方向と表面にとが、±１０°以下の角度である結晶粒の短軸方向長さを５μｍ以下とすることを特徴とする（１）〜（３）のいずれか１つに記載の鋼構造製品の環境助長割れ抵抗性向上方法。
（５）前記超音波衝撃処理を施す前に、前記鋼構造製品の環境助長割れが問題となる箇所及びその近傍箇所に、前処理を施すことを特徴とする（１）〜（４）のいずれか１つに記載の鋼構造製品の環境助長割れ抵抗性向上方法。
（６）前記前処理が、前記鋼構造製品の環境助長割れが問題となる箇所及びその近傍箇所の内部応力および／または表面応力を変化させる処理であることを特徴とする（５）に記載の鋼構造製品の環境助長割れ抵抗性向上方法。
（７）前記前処理が、前記金属構造製品の環境助長割れが問題となる箇所の亀裂を検出すると共に、検出された亀裂を除去する処理を含むことを特徴とする（５）又は（６）に記載の金属構造製品の環境助長割れ抵抗性向上方法。
（８）前記超音波衝撃処理が、さらに、前記鋼構造製品の環境助長割れが問題となる箇所の表面形状を応力集中係数が２以下となる形状とし、かつ表面近傍に圧縮残留応力を付与することを特徴とする（１）〜（７）のいずれか１つに記載の鋼構造製品の環境助長割れ抵抗性向上方法。
（９）前記鋼構造製品の環境助長割れが問題となる箇所に、超音波衝撃処理を施し、その後さらに、品質保証検査をすることを特徴とする（１）〜（８）のいずれか１つに記載の鋼構造製品の環境助長割れ抵抗性向上方法。
（１０）前記品質保証検査は、超音波衝撃処理後の処理面が処理前に比べて、５０μｍ以上の厚さが塑性変形していること、および処理面が応力集中係数が２以下となる表面形状となっていることのいずれか一方又は双方を確認するものであることを特徴とする特徴とする（９）に記載の鋼構造製品の環境助長割れ抵抗性向上方法。
（１１）前記品質保証検査の塑性変形の確認は、超音波衝撃処理後の処理面をスンプ法により観察し、処理していない部分に比べてその５０％以上の結晶粒が微細化しているかどうかを判断することによるものであることを特徴とする（１０）に記載の鋼構造製品の環境助長割れ抵抗性向上方法。
（１２）前記品質保証検査の塑性変形の確認は、超音波衝撃処理後の処理面の結晶粒度を超音波粒径測定装置により測定し、処理していない部分に比べてその５０％以上の結晶粒が微細化しているかどうかを判断することによるものであることを特徴とする（１０）に記載の鋼構造製品の環境助長割れ抵抗性向上方法。
（１３）前記品質保証検査の応力集中が生じ難い表面形状の確認は、超音波衝撃処理後の処理面を型取材を用いて型取りし、型取りした面が応力集中の生じ難い表面形状であるかどうかを判断することによるものであることを特徴とする（１０）に記載の鋼構造製品の環境助長割れ抵抗性向上方法。
（１４）前記品質保証検査の応力集中のし難い表面形状の確認は、超音波衝撃処理後の処理面を変位計を用いて測定し、その変位が応力集中の生じ難い面の変位の範囲内であるかどうかを判断することによるものであることを特徴とする（１０）に記載の鋼構造製品の環境助長割れ抵抗性向上方法。
【００１２】
【発明の実施の形態】
本発明が対象とする鋼構造製品は、鋼により構成される機械部品や配管、容器などの構造部品、或いはこれらを組み合わせた装置などの構造物を含むものである。そしてこれらの構造製品は、一般に、鋼材料に切削、曲げなどの加工、或いはさらに、溶接加工を施して製造される。また、鋼材質としては、炭素鋼、低合金鋼、ステンレス鋼などの鋼材である。
【００１３】
ところで、環境助長割れは、鋼構造製品が引張応力の存在下で腐食環境にあると、表面或いは表層で生じた微小の亀裂が進展し、大きな割れとして鋼構造製品の機能を低下させるものであるが、この亀裂の進展方向は、図１（ａ）に示すように、引張応力（残留応力、外部応力）に垂直な方向で、通常粒界に沿っている。従って、図１（ｂ）に示すように、引張応力の方向が、粒界の方向と実質的に平行であれば、この応力は亀裂の先端をさらに開口させるようには作用しないので、進展を遅らせ、すなわち亀裂の伝播抵抗を向上させ、環境助長割れを抑制することができる。
【００１４】
本発明者らは、この点に着目し、環境助長割れが問題となる箇所の表層組織を層状組織とすることに想到したものである。層状組織とすることにより、結晶粒界の殆どが引っ張り応力の方向と実質的に平行となるため、上述のとおり、微小亀裂が生じても、亀裂の伝播抵抗性を高めることができ、環境助長割れを抑制することができる。
【００１５】
このように、所要の箇所の表層を層状組織とする手段として、超音波で先端のハンマー部を振幅３０〜６０μｍ、周波数２５〜６０ｋＨｚ、出力０．２〜３ｋＷで振動させる装置により鋼表面を打撃してピーニングを行なう超音波衝撃処理（例えば特許文献１、非特許文献１参照）が好適である。この処理方法は、基本的にはハンマーピーニングと同じであるが、一回一回の打撃のエネルギーは小さいかわりに、１秒間に１万回を超える回数の打撃を与えることによって、鋼に塑性変形を与えるものである。このとき、一回一回の打撃力は小さいために、打撃装置に生じる反動は殆どなく、ハンマーピーニング装置に比べて使用性、施工性の面で優れている。
【００１６】
また、１回の打撃エネルギーが小さいため、先端部のハンマー形状は、小型にすることができ、溶接部や接続部などの微小な部分や狭隘な部分に対しても打撃処理を施すことができる。この点において、環境助長割れが問題となる箇所が小さな部分でも処理が適用可能となる。この場合でも、上述のように打撃回数を極めて多くできることから、十分な塑性変形を与えることができる。
【００１７】
また、この超音波衝撃処理は、鋼表面に対して非常に多くの回数の打撃を与えているので、鋼表面に対して従来のハンマーピーニングにはない効果をもあり、また、一回一回の打撃エネルギーショットは、ショットピーニングよりも大きいので、従来のショットピーニングにない効果もある。
【００１８】
すなわち、先ず、打撃の回数が多いことで、処理の均一性が得られる。ハンマーピーニングでも数パスを同一線上で実施すればある程度の均一性が得られるが、超音波衝撃処理の打撃周波数は、２５〜６０ｋＨｚであり、その得られる均一性はハンマーピーニングのそれとは全く異なるレベルにあり、処理スピードが０．５ｍ／分程度であれば、所要の鋼表面のほとんどを均一にかつ欠陥を残すことなく仕上げることができる。
【００１９】
また、処理後の鋼表面を平滑にするとともに、鋼表層の鋼組織が微細にする作用を有しており、極めて有利である。
【００２０】
発明者らは、鋼材の表面に１．５ｍｍの曲率半径を有する先端ハンマーを有する超音波衝撃装置により、振幅５０μｍ、周波数２５ｋＨｚにて処理速度０．５ｍ／ｍｉｎで１パスの超音波衝撃処理を行ない、処理前後の表層組織を詳細に調査した。その結果を処理前後の鋼材の断面状況として、図２及び図３の（ａ）（ｂ）に、それぞれの組織写真及び模式図で示す。これらの図から判るように、処理面の断面は、超音波衝撃処理により塑性変形し、長軸が表面に実質的に平行に伸展した結晶粒が厚さ方向に多層に並んだ層状組織となっている。このような層状組織では、結晶粒の長軸が表面に実質的に平行になっており、腐食環境に曝される鋼材表面から伸展する亀裂の主たる進展方向である結晶粒界と引張応力の作用する方向とが近接するため、応力腐食割れが軽減されることが考えられる。
【００２１】
そこで、発明者らはこれを確認するために、表１に示す組成を有する厚さ１２ｍｍ鋼板に対して、表２に示すように処理条件を変えて超音波衝撃処理を実施し、処理前後の表層部の組織を調査すると共に、図４に示すビードオンプレートで溶接部を形成する方法で、応力腐食試験片を各水準ごとにそれぞれ３個採取し、環境助長割れ試験を実施した。その組織の性状と試験の結果を表３に示す。
【００２２】
表３から判るように、表面に平行な表層の層状組織の厚さが表面から５０μｍ未満では、環境助長割れ感受性が高く、割れが発生しやすいことがわかる。一方、層状組織の厚さが５０μｍ以上であると、割れが発生することがなく、優れた耐環境助長割れ性を示すことがわかる。
【００２３】
これは、超音波衝撃処理によって表層が、長軸が表面に実質的に平行にな結晶粒で形成された層状組織となることによって、結晶粒界の殆どが応力の方向と実質的に平行な方向に伸びることとなり、表面から粒界に沿って進展することが多い亀裂の進展経路が長くなり、亀裂が板厚方向の深部に達して破断に到るまでの時間が長くなるためと考えられる。なお、実質的に平行とは、層状組織の結晶粒の長軸の方向と表面とが、±１０°以下の角度であることを言う。また、長軸、短軸は、鋼の厚さ方向断面における結晶粒の長軸、短軸をいう。
【００２４】
この層状組織は、層状組織の結晶粒の長軸方向長さと短軸方向長さとの比が５以上であることが好ましい。これは、上述と同様に、結晶粒が表面に平行な長軸方向に伸展したことによって、応力の方向に平行な結晶粒界がより長くなり、亀裂の進展経路が長くなって破断に到るまでの時間が長くなるためである。
【００２５】
さらに、長軸方向長さと短軸方向長さとの比を５以上とすることによって層状組織が均一に形成され、かつ多層に形成することができるため耐環境助長割れに対して極めて有利である。
【００２６】
また、この層状組織は、層状組織の結晶粒の短軸方向長さが５μｍ以下であることが好ましい。短軸方向の長さを５μｍ以上では、層状組織の形成が不十分であり、破断までの時間がやや短くなる。一方、５μｍ以下であると、破断までの時間をより十分に確保できる。
【００２７】
さらに、この超音波衝撃処理は、塑性変形により鋼表層を層状組織とすることができると共に、表面形状をなめらかな平面形状とし、かつ表層近傍に圧縮残留応力を付与することができる。
【００２８】
したがって、好ましくは、超音波衝撃処理によって環境助長割れが問題となる箇所の鋼表層を層状組織とすると共に、この箇所の表面を応力集中の生じ難い表面形状とし、かつ表面近傍に圧縮残留応力を付与することが好ましい。応力集中の生じ難い表面形状とは、例えば、溶接止端部の場合、応力集中係数が２以下となるような形状であって、このような表面形状とすることによって応力集中が生じ難くなり、かつ表面近傍、例えば表面から５０μｍ以内の範囲、に圧縮残留応力が付与されることによって、環境助長割れの起点となる微小な欠陥が大きな亀裂に伸展することを抑制することができるので、これらの微小亀裂を無害化し、さらに、環境助長割れ抵抗性を向上させることができる。
【００２９】
以上のように、鋼材料の表面に超音波衝撃処理を施すことによって、その表層部を層状組織とし、あるいは、さらに、表面を応力集中の生じ難い形状とすると共に表面近傍に圧縮残留応力を付与することによって、応力によって助長される、応力腐食割れ、水素誘起割れ、硫化物応力腐食割れなど、各種の環境助長割れを抑制、低減することができ、耐環境助長割れ性に優れた鋼構造製品とすることができる。
【００３０】
この超音波衝撃処理は、前述の鋼構造製品の少なくとも環境助長割れが問題となる箇所に施せばよく、その問題となる箇所とは、鋼構造製品で腐食環境と接し、かつ応力が負荷ないし残留する個所である。応力が集中ないし残留する具体的な箇所として、溶接継手部（溶接ボンド部、溶接熱影響部）が先ず挙げられる。鋼構造製品の多くが溶接を伴って製作され、その溶接継手部には残留応力が発生する。また、溶接継手部の溶接止端部は、応力が集中しやすい。
【００３１】
従って、鋼構造製品の溶接部、すなわち、溶接ボンド部および／または溶接熱影響部を含む部分を超音波衝撃処理することが好ましく、さらには、溶接止端部を含めることも好ましい。
【００３２】
溶接部以外に、応力が集中ないし負荷される箇所の例としては、鋼構造製品を作成する段階で加えられることのある、鋸断、せん断、溶断などによる切断箇所がある。これらの箇所は、切断に伴なって端面に大きな引張応力、せん断応力が負荷される。そのほか、鋼構造製品には、曲げや捻りを加えて構成されることがあり、これらが集中する箇所には、これらの曲げやねじりに伴う引張応力が負荷されている。これらの加工過程で生じる応力のほか、使用状態で外部から応力が負荷される箇所もあり、これらも本処理の対象となる。このように引張応力が負荷されている箇所が、腐食環境下にあると、上述のとおり、環境助長割れを発生させることとなる。
【００３３】
上述のように、環境助長割れの発生は、環境、応力及び材料の３つの条件が関与する。本発明の超音波衝撃処理は、このうちの応力条件を低減することを主眼とするものであり、特に、鋼構造製品の材料を限定するものではないが、環境助長割れは、強度、硬度の高い材料に発生しやすいと言う観点から、引張強度が４９０Ｎ／ｍｍ^２以上の鋼材からなる構造製品の必要箇所には少なくとも施すことが好ましい。引張強度が４９０Ｎ／ｍｍ^２以上の鋼材では、溶接部の残留応力がより高くなるために、環境助長割れ感受性が一段と高くなる。このため、引張強度が４９０Ｎ／ｍｍ^２以上の鋼材の溶接部には、超音波衝撃処理を施すことが一段と有効であるとともに、超音波衝撃処理を施す効果がより大きい。超音波衝撃処理を施す効果は材料の強度が高くなるとともにより大きくなるので、引張強度が５９０Ｎ／ｍｍ^２以上の鋼材の溶接部、引張強度が６９０Ｎ／ｍｍ^２以上の鋼材の溶接部、引張強度が７８０Ｎ／ｍｍ^２以上の鋼材の溶接部、引張強度が９８０Ｎ／ｍｍ^２以上の鋼材の溶接部、と強度が高くなるのにしたがって、超音波衝撃処理を施す効果と必要性が大きくなる。
【００３４】
上述のように超音波衝撃処理は、先端部に所定の曲率半径を有する先端ハンマーを有する超音波衝撃装置により、振幅３０μｍ以上６０μｍ以下でサイクル数４０ｋＨｚ以上６０ｋＨｚ以下、又は、振幅５０μｍ以上６０μｍ以下でサイクル数２５ｋＨｚ以上４０ｋＨｚ未満、いずれかの条件にて必要の時間、所要の鋼表面部分に対して行うが、この衝撃により表層部分を塑性変形させ、その結晶粒を表面に実質的に平行な層状組織とするとともに、この塑性変形により、好ましくは、応力集中の発生し難い表面形状とし、かつ残留圧縮応力付与することができ、環境助長割れ抵抗性を高めることができる。
【００３５】
このためには、超音波衝撃処理による鋼表層の塑性変形の厚さは、５０μｍ以上であることが必要である。これ未満では、表層の層状組織を５０μｍ以上確保することが困難であり、十分な耐環境助長割れ特性を得ることが困難となる。また、引張応力を解消し圧縮応力を付与する点からも、表面から５０μｍ以上の厚さを塑性変形させ、層状組織とすることが必要である。しかしながら、この表層の層状組織或いは塑性変形の厚さを過度に大きくすると、表層が過度に硬化したり、変形が大きくなり過ぎたりして製品としての表面性状が悪くなる一方、処理のためのコストが増えるために好ましくない。
【００３６】
所要の厚さの層状組織或いは塑性変形を得るために必要な変形のためのエネルギーはほぼ一定であるため、１サイクルの衝撃エネルギーを大きくして短時間に処理しても良いが、均一性を高めたい場合や、衝撃部位の位置をより精緻に制御し、過度な塑性変形を防止したい場合は、１サイクルの衝撃エネルギーを小さくし、二回以上の処理を同一箇所に対して行なうことが好ましい。
【００３７】
また、衝撃エネルギーによって生じる層状組織或いは塑性変形の厚さは、衝撃装置の先端のハンマーの曲率半径Ｒとも関係しており、１サイクルの衝撃エネルギーが同じでも、Ｒが小さければ、１サイクルの衝撃で生じる層状組織或いは塑性変形の厚さは大きくなり、Ｒが大きければその厚さは小さくなる。
【００３８】
また、表面を応力集中し難い形状とし、圧縮残留応力を付与する場合は、ハンマーのＲが小さければ、１サイクルで形成される表面形状の範囲が狭いので繰り返し処理が必要となり、またＲが大きければ、形状の制御が困難となることもある。従って、超音波打撃処理装置の先端のハンマーの形状は、処理対象とする鋼構造製品の状況によって適宜選択する。
【００３９】
超音波衝撃処理を施すにあたっては、鋼構造製品の処理対象箇所の表面から所要の厚さを層状組織とし、或いはさらに応力集中し難い形状とし圧縮残留応力を付与するために必要なハンマーの形状、１サイクルの打撃エネルギー、パス数、処理回数などの処理条件を、例えば、鋼材質或いは必要により、溶接部、切断端面などの処理箇所ごとに予備試験などにより、予め決めて置くことによって、処理後に所要の層状組織或いは圧縮残留応力を付与することができる。
【００４０】
ところで、本発明の環境助長割れ抵抗性の向上方法においては、鋼構造製品の超音波衝撃処理を施す箇所に対して、この箇所の内部応力および／または表面応力など応力状態を変化させるような処理を、超音波衝撃処理を施した後には行なわないようにすることが好ましい。
【００４１】
すなわち、超音波打撃処理を施して、当該箇所の表層を層状組織とし、或いは塑性変形させて表面形状を応力集中の生じ難い形状とし、かつ残留圧縮応力を付与した後で、当該箇所及びその近傍箇所の表層の組織、塑性変形状況、応力状態などを変化させるような処理、例えば、塑性加工、矯正、熱処理、溶接などを施すと、超音波衝撃処理により形成された環境助長割れを抑制するための上記の表層の性状がこれによって減殺され、抑制効果が低下する。
【００４２】
従って、本発明の超音波衝撃処理方法においては、鋼構造製品の少なくとも当該処理を施す箇所に対しては、例えば、塑性加工、矯正、熱処理、溶接など、当該箇所の表層の組織、塑性変形状況、応力状態などを変化させるような処理は、超音波衝撃処理を施す前に、前処理として施しておくことが好ましく、超音波衝撃処理後は、このような処理を行なわないようにすることが好ましい。
【００４３】
また、上記の前処理においては、上述の各処理のほか、環境助長割れが問題となる箇所に対する亀裂の有無を検査し、検出された亀裂を除去する処理を含むことが好ましい。すなわち、目視検査、浸透探傷検査、磁粉探傷検査、渦流探傷検査など鋼構造製品の亀裂を検査する適切な手段により、環境助長割れが問題となる箇所、すなわち、超音波衝撃処理を施そうとする箇所に対して亀裂の有無を検査し、そして、検出された亀裂に対して、事前にこれを除去する処理を施すものである。除去する方法は、亀裂部分をグラインダー、切削工具等により研削・切削して除去する方法、或いは溶接により亀裂部を溶融接着する方法など適宜な方法を採用しうる。
【００４４】
また、特に、除去した亀裂の深さが３ｍｍ以上である場合は、亀裂部分を研削除去し、肉盛溶接を行った後、この箇所の表面をグラインダー、切削工具等の機械的手段により平滑な形状に仕上げ、さらに上述の亀裂の検査処理によって亀裂が検出されないことを確認する処理を含むことが好ましい。
【００４５】
本発明においては、必要に応じて上述の前処理を施した後、上記の超音波衝撃処理を施し、その後、必要に応じて、品質保証検査を行なう。
【００４６】
超音波衝撃処理後の品質保証検査は、処理面が処理前と比較して５０μｍ以上の厚さまで塑性変形していること、すなわち、表面から５０μｍ以上の厚さの表層が層状組織となっていること、および処理面が応力集中の生じ難い表面形状となっているかどうかのいずれか一方又は双方を確認するものである。
【００４７】
処理面が処理前と比較して５０μｍ以上の厚さまで塑性変形していることを確認するには、スンプ法により処理面の複製を制作し、その結晶組織を観察するか、あるいは処理面の結晶粒度を超音波粒径測定装置によるかのいずれかにより結晶粒度を測定し、処理していない部分に比べて５０％以上の結晶粒が微細化しているかどうかを判断することによって行なうことができる。結晶粒の微細化が５０％未満では、層状組織の形成が不十分である。なおここでいう微細化とは、処理を施していない部分の結晶粒度より粒度番号で１以上大きければ微細化されていると判断する。
【００４８】
また、処理面が応力集中し難い表面形状となっているかどうかを確認するには、例えば歯科用形象材のような型取り材を用いて型取りし、型取りした複製の表面形状を検査するか、或いは、レーザー変位計などの高精度な変位測定装置を用いて表面の変位を測定することによって、処理面が応力集中のし難い表面の曲率ないしは変位を有するかどうかを判断することによって行なうことができる。
【００４９】
以上のような方法により、超音波衝撃処理後の表層組織或いは、表面形状を確認する品質保証検査を行なうことによって、鋼構造製品の耐環境助長割れが問題となる箇所の環境助長割れ抵抗性の向上を確認することができる。
【００５０】
なお、この品質保証検査により、所要の表面形状或いは表層組織が得られていない場合は、超音波衝撃処理を繰り返し、所要の表面性状、表層組織となるようにすることはいうまでもない。
【００５１】
【実施例】
以下、実施例により、本発明を説明する。
【００５２】
表１に示す組成の鋼（板厚１２ｍｍ）を母材とし、共金系の溶接材料を用いてアーク溶接した溶接部を試験体とし、この箇所に超音波衝撃処理を付与して表層の鋼組織を、長軸が表面に実質的に平行な結晶粒からなる層状組織とした。比較材として溶接ままの試験体を用いた。図４に概略を示す要領で、幅１００ｍｍ、長さ２００ｍｍ、板厚は元厚まま、の試験片の中央に、ビードオンプレートで溶接部を形成し、そのまま環境助長割れ試験片とした。言うまでもなく、溶接ままの試験片の溶接部には溶接残留応力が存在している。
【００５３】
これらの試験片を、９０℃の１ＮＮａ_２ＣＯ_３＋１ＮＮａＨＣＯ_３水溶液中に浸漬し、ポテンシオスタットを用いて一定の電位（標準水素電極基準に換算して−０．４２Ｖ）に保持し、９０日試験した。試験後の試験片は、浸透試験および断面観察によって環境助長割れの発生の有無を確認した。
【００５４】
表２に超音波衝撃処理条件を示し、表３には結晶粒および環境助長割れ試験結果を示す。
【００５５】
表３から明らかな通り、表層の鋼組織を、長軸が表面に実質的に平行な結晶粒からなる層状組織とした本発明例１〜８では環境助長割れ（応力腐食割れ）がまったく発生していないのに対して、溶接ままあるいは処理が不充分な比較例９〜１２では環境助長割れ（応力腐食割れ）が発生しており、本発明の効果が明らかである。
【００５６】
【表１】

【００５７】
【表２】

【００５８】
【表３】

【００５９】
【発明の効果】
本発明の方法によれば、鋼構造製品の環境助長割れの問題となる箇所に超音波衝撃処理が施され、表層が層状組織となり、さらに好適には、表面が応力集中が生じ難い表面形状で、かつ残留圧縮応力が付与されているため、腐食環境に曝されても微小亀裂が生じ難くまた、微小亀裂が存在しても亀裂の厚さ方向への進展が抑制され、破断時間が大幅に伸び、環境助長割れに対して優れた抵抗性を有する鋼構造製品を得ることができる。また、超音波衝撃処理後の品質保証検査を組み合わせることによって、処理を施した箇所の表層が所定の層状組織となり、さらには表面形状となっていることを確認できるため、鋼構造製品の所要箇所の環境助長割れ抵抗性を確実に向上させることができる。
【図面の簡単な説明】
【図１】応力腐食割れ亀裂の進展状況を示す模式図であり、（ａ）は、粒界が引張応力の方向に垂直な方向にある場合、（ｂ）は、粒界のほとんどが引張応力の方向と平行する方向にある場合である。
【図２】鋼構造製品の超音波衝撃処理前の組織を示す図であり、（ａ）は、組織写真（ｂ）は、その模式図である。
【図３】鋼構造製品の超音波衝撃処理後の組織を示す図であり、（ａ）は、組織写真（ｂ）は、その模式図である。
【図４】応力腐食試験片の採取状況を示す図である。
【符号の説明】
１…母材
２…溶接部[0001]
BACKGROUND OF THE INVENTION
The present invention relates to various equipment such as chemical plants and petroleum refining equipment, pipes, or steel structures or structural products used as gas pipelines, and various kinds such as chloride, amine, hydrogen sulfide, nitrate, carbonate, etc. Increases resistance to stress corrosion cracking and hydrogen embrittlement cracking (hereinafter referred to as environmentally assisted cracking) in corrosive environments The ring The present invention relates to a method of improving resistance to Sakai long crack.
[0002]
[Prior art]
Environmentally-assisted cracking: stress corrosion cracking of structures and piping in the low concentration chloride aqueous solution or in the vicinity of the coast where chlorides are concentrated and diluted repeatedly, heat exchange of the reaction tower and heating furnace of the hydrodesulfurization equipment in the petroleum refining industry Intergranular stress corrosion cracking due to polythionic acid in the heat affected zone of austenitic stainless steel welding equipment, etc., stress corrosion cracking in a nitrate solution environment of carbon steel and low alloy steel in ammonium nitrate production equipment, oil well pipes, hot air furnaces, etc., natural Stress corrosion cracking of carbon steel in a carbonate aqueous solution environment in gas transport pipelines, high pressure gas turbines, storage of fertilizer ammonia, stress corrosion cracking of welded heat affected zone of carbon steel in transport containers, carbon steel in chemical plant CO-CO ₂ -H ₂ There is stress corrosion cracking in an O environment, and Ni alloy also has stress corrosion cracking of Ni alloy in a pressure vessel, etc. Other Cu and Cu alloys are also in a humid atmosphere, seawater, freshwater environment, In the Al alloy, stress corrosion cracking occurs in an aqueous chloride solution environment.
[0003]
In addition, there are cracks due to stress generated by penetration of hydrogen into the metal material, that is, hydrogen-induced cracks. The occurrence of the crack is greatly influenced by the hydrogen penetration reaction rate, but the growth of the crack is accelerated by the load or residual stress on the metal material, and the same behavior as the above-described stress corrosion cracking is obtained. That is, these cracks are caused by the development and promotion of minute cracks on the surface or surface layer of the metal structure product in the presence of stress by the corrosive environment.
[0004]
These cracks are generated by superimposing three conditions of the material, the corrosive environment, and the presence of stress, and the generation of cracks can be suppressed by removing any of these conditions. Change from the material side to a material that does not cause stress corrosion cracking, for example, change from austenitic stainless steel to ferritic stainless steel, from the aspect of corrosive environment, adopting anti-corrosion measures such as cathodic protection, or from the aspect of stress Various measures such as heat treatment (stress relief annealing) and peening treatment for removing or reducing load stress have been conventionally performed.
[0005]
However, the change of the material is limited when an appropriate selection can be made due to the restriction on the material strength and the restriction on the combination with the corrosive environment. In addition, when the object to be processed is a large structure product, the stress relief annealing requires a large heat treatment furnace for heat treatment, and heat treatment itself is impossible for a fixed structure or the like. In addition, depending on the material, a change in the material due to the stress-relieving annealing heat treatment is also a limitation. Furthermore, this heat treatment has no effect on externally applied stress. Although reduction of residual stress by shot peening is also effective, as with heat treatment, large metal structure products have problems on equipment.
[0006]
As described above, these conventional measures are limited in application and cost is high, so it is difficult to be an economical and efficient measure.
[0007]
In addition, the processing method by the ultrasonic impact energy for improving the intensity | strength of the welding part of a welded product by the above-mentioned peening process and forming the stress pattern which suppresses stress concentration and a microstress defect is proposed (for example, patent documents) 1). It is also known that the surface is smoothed by this impact treatment (see, for example, Non-Patent Document 1).
[0008]
However, these documents do not mention environmentally assisted cracking.
[0009]
[Patent Document 1]
US Pat. No. 6,171,415
[Non-Patent Document 1]
Surface Nanocrystallization (SNC) of metallic Materials-Presentation of the Concept behind a New Approach, Journal al. Sci. Technol. Vol. 15 No. 3, 1999)
[0010]
[Problems to be solved by the invention]
The present invention solves the above problems. ,ring It is an object of the present invention to provide a method for improving the Sakai aid crack resistance.
[0011]
[Means for Solving the Problems]
The present invention is made in order to solve the above-described problems, and the steel surface is made by using a tool that vibrates the tip with an ultrasonic wave with an amplitude of 30 to 60 μm, a frequency of 25 kHz to 60 kHz, and an output of 0.2 to 3 kW. By applying ultrasonic impact treatment to the places where environmentally assisted cracking of steel structure products is a problem, the structure of the steel surface layer is improved, thus obtaining a steel structure product with excellent environmentally assisted cracking resistance. In addition, an appropriate pre-processing for performing this processing and an inspection after the processing are performed to ensure the effect. The gist is as follows.
(1 )steel The ultrasonic shock device having a tip hammer having a radius of curvature of 1 to 2 mm at the tip at a location where environmentally assisted cracking becomes a problem because the stress of the structured product concentrates or remains, the tip of the tip has an amplitude of 30 μm to 60 μm, Vibrating under any condition of cycle number 40 kHz to 60 kHz, output 0.2 to 3 kW, amplitude 50 μm to 60 μm, cycle number 25 kHz to less than 40 kHz, output 0.2 to 3 kW, and hitting the processing target part The surface of the steel structure with a thickness of 50 μm or more from the surface is subjected to ultrasonic impact treatment, and the major axis direction of the crystal grains and the surface are at an angle of ± 10 ° or less, and the major axis length of the crystal grains is A method for improving the environment-assisted cracking resistance of a steel structure product, characterized in that the ratio of the length in the minor axis direction is 5 or more to form a layered structure composed of crystal grains.
( 2 ) The steel where the environmentally assisted cracking of the steel structure product becomes a problem has a tensile strength of 490 N / mm. ² It is characterized by being steel of grade or better ( 1 ) Method for improving environment-assisted cracking resistance of steel structure products as described in 1).
( 3 ) The location where the environmentally assisted cracking of the steel structure product becomes a problem includes a weld bond portion and / or a weld heat affected zone ( 1) Ma ( 2 ) Improvement method of environmentally-assisted cracking resistance of steel structure products described in 1.).
( 4 ) The major axis direction of the crystal grains of the layered structure and the surface have an angle of ± 10 ° or less, and the minor axis length of the crystal grains is 5 μm or less ( 1 ) ~ ( 3 ) A method for improving the environment-assisted cracking resistance of a steel structure product according to any one of the above.
( 5 ) Before performing the ultrasonic impact treatment, the steel structure product is subjected to pretreatment at a location where environmentally assisted cracking is a problem and in the vicinity thereof ( 1 ) ~ ( 4 ) A method for improving the environment-assisted cracking resistance of a steel structure product according to any one of the above.
( 6 ) The pretreatment is a treatment for changing the internal stress and / or the surface stress at a location where the environmentally assisted cracking of the steel structure product is a problem and its vicinity. 5 ) Method for improving environment-assisted cracking resistance of steel structure products as described in 1).
( 7 ) The pretreatment includes a process of detecting a crack where the environmentally assisted crack of the metal structure product is a problem and removing the detected crack ( 5 Or ( 6 ) Environmentally assisted cracking resistance improving method for metal structure products as described in the above).
( 8 ) The ultrasonic impact treatment further provides a surface shape where the stress-enhanced cracking of the steel structure product is a problem and a stress concentration factor of 2 or less, and imparts compressive residual stress in the vicinity of the surface. Characterize ( 1 ) ~ ( 7 ) A method for improving the environment-assisted cracking resistance of a steel structure product according to any one of the above.
( 9 ) It is characterized in that ultrasonic impact treatment is applied to the location where environmentally assisted cracking of the steel structure product becomes a problem, and then further quality assurance inspection is performed ( 1 ) ~ ( 8 ) A method for improving the environment-assisted cracking resistance of a steel structure product according to any one of the above.
( 10 ) In the quality assurance inspection, the treated surface after the ultrasonic impact treatment is plastically deformed with a thickness of 50 μm or more compared to before the treatment, and the treated surface has a surface shape with a stress concentration factor of 2 or less. It is characterized by confirming either one or both of 9 ) Method for improving environment-assisted cracking resistance of steel structure products as described in 1).
( 11 ) Confirmation of plastic deformation in the quality assurance inspection is made by observing the treated surface after ultrasonic impact treatment by the sump method and judging whether or not 50% or more of the crystal grains are finer than the untreated portion. It is characterized by ( 10 ) Method for improving environment-assisted cracking resistance of steel structure products as described in 1).
( 12 ) Confirmation of plastic deformation in the quality assurance inspection is carried out by measuring the crystal grain size of the treated surface after ultrasonic impact treatment with an ultrasonic grain size measuring device, and 50% or more of the crystal grains are compared with the untreated part. It is characterized by judging whether or not it is miniaturized ( 10 ) Method for improving environment-assisted cracking resistance of steel structure products as described in 1).
( 13 ) Confirmation of the surface shape where stress concentration does not easily occur in the quality assurance inspection is to check whether the processed surface after ultrasonic impact treatment is molded using a mold material, and the surface that has been subjected to molding has a surface shape where stress concentration is unlikely to occur. Characterized by judging whether or not ( 10 ) Method for improving environment-assisted cracking resistance of steel structure products as described in 1).
( 14 ) Confirmation of the surface shape that is difficult to concentrate stress in the quality assurance inspection is performed by measuring the treated surface after ultrasonic shock treatment using a displacement meter, and the displacement is within the range of the surface where stress concentration is unlikely to occur. It is characterized by judging whether or not ( 10 ) Method for improving environment-assisted cracking resistance of steel structure products as described in 1).
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Intended for the present invention steel Structural products steel It includes structural parts such as machine parts, pipes, structural parts such as containers, or a combination of these. And these structural products are generally steel Manufactured by subjecting the material to machining such as cutting and bending, or further welding. Also, steel The material is carbon steel, low alloy steel, stainless steel, etc. With wood is there.
[0013]
By the way, environmentally friendly cracking steel If a structural product is in a corrosive environment in the presence of tensile stress, microcracks that have developed on the surface or surface will develop and become large cracks. steel Although it lowers the function of the structural product, the direction of crack growth is perpendicular to the tensile stress (residual stress, external stress) and usually along the grain boundary as shown in FIG. Yes. Accordingly, as shown in FIG. 1 (b), if the direction of the tensile stress is substantially parallel to the direction of the grain boundary, this stress does not act to further open the crack tip, and therefore the progress is made. Delay, that is, the propagation resistance of cracks can be improved and environmentally assisted cracking can be suppressed.
[0014]
The present inventors pay attention to this point, and have conceived that the surface layer structure where the environmentally assisted cracking is a problem is a layered structure. By adopting a lamellar structure, most of the crystal grain boundaries are substantially parallel to the direction of tensile stress, and as described above, even if a microcrack occurs, the propagation resistance of the crack can be improved, and the environment is promoted. Cracking can be suppressed.
[0015]
In this way, as a means to make the surface layer of the required place a layered structure, the tip of the hammer part is amplituded with ultrasound. 30 ~ 60μm, frequency 25 Ultrasonic impact treatment (for example, see Patent Document 1 and Non-Patent Document 1) in which peening is performed by hitting the steel surface with a device that vibrates at ˜60 kHz and an output of 0.2 to 3 kW is suitable. This treatment method is basically the same as hammer peening, but the plastic deformation of the steel is achieved by giving more than 10,000 shots per second, instead of the energy of each shot being small. Is to give. At this time, since the impact force per time is small, there is almost no reaction that occurs in the impact device, which is superior in terms of usability and workability compared to the hammer peening device.
[0016]
In addition, since the impact energy at one time is small, the hammer shape at the tip can be reduced in size, and a striking process can be performed on a minute part or a narrow part such as a welded part or a connecting part. . In this respect, the processing can be applied even in a small portion where environmentally assisted cracking is a problem. Even in this case, since the number of hits can be extremely increased as described above, sufficient plastic deformation can be provided.
[0017]
In addition, this ultrasonic shock treatment steel Because it hits the surface so many times, steel There is also an effect that the conventional hammer peening does not have on the surface, and since each shot energy shot is larger than the shot peening, there is also an effect that the conventional shot peening does not have.
[0018]
That is, first, the uniformity of processing can be obtained because the number of hits is large. Even with hammer peening, if you perform several passes on the same line, a certain degree of uniformity can be obtained, but the impact frequency of ultrasonic impact treatment is 25 The uniformity obtained is at a level completely different from that of hammer peening, and if the processing speed is about 0.5 m / min, most of the required steel surface is uniform and without leaving any defects. Can be finished.
[0019]
Also after processing steel While smoothing the surface, steel Surface steel The tissue has the effect of making it fine, which is extremely advantageous.
[0020]
The inventors performed an ultrasonic impact treatment of one pass at a processing speed of 0.5 m / min at an amplitude of 50 μm and a frequency of 25 kHz using an ultrasonic impact device having a tip hammer having a radius of curvature of 1.5 mm on the surface of the steel material. The surface structure before and after the treatment was investigated in detail. The results are shown in FIG. 2 and FIG. 3 (a) and FIG. 3 (b) as structural images and schematic views of the steel material before and after the treatment. As can be seen from these figures, the cross section of the treated surface is plastically deformed by ultrasonic impact treatment, and has a layered structure in which crystal grains whose major axis extends substantially parallel to the surface are arranged in multiple layers in the thickness direction. ing. In such a lamellar structure, the long axis of the crystal grains is substantially parallel to the surface, and the action of the grain boundaries and tensile stress, which are the main growth directions of cracks extending from the steel surface exposed to the corrosive environment. It is conceivable that stress corrosion cracking is reduced because of the close proximity to the direction in which it is applied.
[0021]
Therefore, in order to confirm this, the inventors have a thickness of 12 mm having the composition shown in Table 1. steel As shown in Table 2, the ultrasonic shock treatment is performed on the plate while changing the treatment conditions, the structure of the surface layer portion before and after the treatment is investigated, and the welded portion is formed with the bead on plate shown in FIG. Then, three stress corrosion test pieces were collected for each level, and an environmentally assisted cracking test was conducted. Table 3 shows the properties of the tissues and the test results.
[0022]
As can be seen from Table 3, when the thickness of the layered structure of the surface layer parallel to the surface is less than 50 μm from the surface, the environmentally assisted cracking sensitivity is high, and it can be seen that cracking is likely to occur. On the other hand, it can be seen that when the thickness of the layered structure is 50 μm or more, cracks do not occur and excellent environmentally-friendly crack resistance is exhibited.
[0023]
This is because most of the crystal grain boundaries are substantially parallel to the direction of stress because the surface layer becomes a layered structure formed by crystal grains whose major axis is substantially parallel to the surface by ultrasonic impact treatment. This is considered to be because the growth path of cracks, which often grow along the grain boundary from the surface, becomes longer, and the time until the crack reaches the deep part in the plate thickness direction and breaks becomes longer. . Note that “substantially parallel” means that the major axis direction of the crystal grains of the layered structure and the surface are at an angle of ± 10 ° or less. The major and minor axes are steel The major axis and the minor axis of crystal grains in the cross section in the thickness direction.
[0024]
In this layered structure, the ratio of the length in the major axis direction to the length in the minor axis direction of the crystal grains of the layered structure is preferably 5 or more. This is because, as described above, the crystal grain extends in the major axis direction parallel to the surface, so that the crystal grain boundary parallel to the stress direction becomes longer, and the crack propagation path becomes longer, leading to fracture. This is because the time until is increased.
[0025]
Furthermore, by setting the ratio of the length in the major axis direction to the length in the minor axis direction to be 5 or more, the lamellar structure can be formed uniformly and can be formed in multiple layers, which is extremely advantageous for resistance to environmentally assisted cracking.
[0026]
Further, in this lamellar structure, it is preferable that the minor axis direction length of crystal grains of the lamellar structure is 5 μm or less. When the length in the minor axis direction is 5 μm or more, the formation of the layered structure is insufficient, and the time until fracture is slightly shortened. On the other hand, when it is 5 μm or less, it is possible to secure a sufficient time to break.
[0027]
Furthermore, this ultrasonic shock treatment is caused by plastic deformation. steel The surface layer can have a layered structure, the surface shape can be a smooth planar shape, and compressive residual stress can be applied in the vicinity of the surface layer.
[0028]
Therefore, preferably, the location where environmentally assisted cracking is a problem due to ultrasonic impact treatment. steel It is preferable that the surface layer has a layered structure, the surface of this portion has a surface shape in which stress concentration hardly occurs, and compressive residual stress is applied in the vicinity of the surface. For example, in the case of a weld toe, the surface shape where stress concentration is unlikely to occur is a shape where the stress concentration coefficient is 2 or less, and by making such a surface shape, stress concentration is less likely to occur, In addition, by applying compressive residual stress in the vicinity of the surface, for example, within a range of 50 μm or less from the surface, it is possible to prevent the minute defects that are the starting points of environmentally assisted cracks from extending into large cracks. Microcracks can be rendered harmless, and further environmentally assisted crack resistance can be improved.
[0029]
As above steel By applying ultrasonic shock treatment to the surface of the material, the surface layer is made into a layered structure, or further, the surface is made into a shape in which stress concentration is unlikely to occur and compressive residual stress is applied to the vicinity of the surface by stress. It is possible to suppress and reduce various environmentally assisted cracks such as stress corrosion cracking, hydrogen induced cracking, sulfide stress corrosion cracking, etc. steel It can be a structural product.
[0030]
This ultrasonic shock treatment is performed as described above. steel What is necessary is just to give it to the place where the environmentally assisted crack of a structural product becomes a problem, and the problem place is steel A structural product that is in contact with a corrosive environment and where stress is applied or remains. As a specific part where stress is concentrated or remains, a welded joint part (weld bond part, weld heat affected part) is first mentioned. steel Many structural products are manufactured with welding, and residual stress is generated in the welded joint. Moreover, stress tends to concentrate on the weld toe of the weld joint.
[0031]
Therefore, steel It is preferable to ultrasonically treat the welded portion of the structural product, that is, the portion including the weld bond portion and / or the weld heat affected zone, and it is also preferable to include a weld toe portion.
[0032]
Examples of places where stress is concentrated or loaded in addition to welds include: steel There are cutting points by sawing, shearing, fusing, etc. that may be added in the stage of creating a structured product. In these places, a large tensile stress and shearing stress are applied to the end face along with the cutting. others, steel A structural product may be configured by bending and twisting, and tensile stress associated with these bending and twisting is applied to a location where these are concentrated. In addition to the stress generated in these processing processes, there are places where stress is applied from the outside in the state of use, and these are also the targets of this processing. As described above, when the portion where the tensile stress is applied is in a corrosive environment, an environmentally assisted crack is generated.
[0033]
As described above, the occurrence of environmentally assisted cracking involves three conditions: environment, stress and material. The ultrasonic impact treatment of the present invention is mainly intended to reduce the stress condition, and in particular, steel Although the material of the structural product is not limited, from the viewpoint that environmentally assisted cracking is likely to occur in a material having high strength and hardness, the tensile strength is 490 N / mm. ² It is preferable to apply at least the necessary portions of the structural products made of the above steel materials. Tensile strength is 490 N / mm ² In the above steel materials, since the residual stress of the welded portion becomes higher, the environmentally assisted cracking sensitivity is further increased. For this reason, the tensile strength is 490 N / mm ² It is more effective to apply ultrasonic shock treatment to the above-described welded steel parts, and the effect of applying ultrasonic shock treatment is greater. The effect of applying ultrasonic impact treatment increases with the strength of the material, so the tensile strength is 590 N / mm ² The above steel welds, tensile strength is 690 N / mm ² The above steel welds, tensile strength is 780 N / mm ² The welded part of the above steel material, tensile strength is 980 N / mm ² As the strength of the above-described welded portion of the steel material increases, the effect and necessity of applying ultrasonic impact treatment increase.
[0034]
As described above, the ultrasonic impact treatment is performed at an amplitude of 30 μm to 60 μm and a cycle number of 40 kHz to 60 kHz, or an amplitude of 50 μm to 60 μm, using an ultrasonic impact device having a tip hammer having a predetermined radius of curvature at the tip. Number of cycles 25 It is performed on the required steel surface part for a required time under any condition of kHz or more and less than 40 kHz, but the surface layer part is plastically deformed by this impact, and the crystal grains are substantially parallel to the surface. At the same time, this plastic deformation can preferably provide a surface shape in which stress concentration is unlikely to occur, and can impart residual compressive stress, thereby enhancing environmentally-assisted crack resistance.
[0035]
For this, by ultrasonic shock treatment steel The thickness of the plastic deformation of the surface layer needs to be 50 μm or more. If it is less than this, it is difficult to secure a layered structure of the surface layer of 50 μm or more, and it becomes difficult to obtain sufficient environment-assisted cracking characteristics. Also, from the viewpoint of eliminating the tensile stress and applying the compressive stress, it is necessary to plastically deform a thickness of 50 μm or more from the surface to form a layered structure. However, if the surface layer structure or the thickness of plastic deformation is excessively large, the surface layer is excessively hardened or the deformation becomes excessively large and the surface properties of the product are deteriorated. Is not preferable because of increasing
[0036]
The energy required for deformation to obtain a layered structure or plastic deformation of the required thickness is almost constant, so the impact energy of one cycle may be increased and processed in a short time. When it is desired to increase or to control the position of the impact site more precisely and to prevent excessive plastic deformation, it is preferable to reduce the impact energy of one cycle and perform two or more treatments on the same location. .
[0037]
The thickness of the lamellar structure or plastic deformation caused by the impact energy is also related to the radius of curvature R of the hammer at the tip of the impact device. Even if the impact energy of one cycle is the same, if R is small, the impact of one cycle The thickness of the lamellar structure or plastic deformation generated in (1) increases, and if R is large, the thickness decreases.
[0038]
In addition, when the surface is made difficult to concentrate stress and compressive residual stress is applied, if the hammer R is small, the surface shape range formed in one cycle is narrow, so repeated processing is required, and R is large. In this case, it may be difficult to control the shape. Therefore, the shape of the hammer at the tip of the ultrasonic hitting processing device is the processing target. steel Select according to the status of the structural product.
[0039]
In applying ultrasonic shock treatment, steel Hammer shape, one cycle of impact energy, number of passes, processing required to apply a compressive residual stress with a layered structure of the required thickness from the surface of the processing target part of a structured product, or to a shape that does not allow stress concentration. Processing conditions such as the number of times, for example steel A predetermined lamellar structure or compressive residual stress can be applied after processing by preliminarily determining and placing the material by necessity, such as a preliminary test, for each processing location such as a welded portion and a cut end surface.
[0040]
By the way, in the method for improving the environment-assisted cracking resistance of the present invention, steel Do not perform processing that changes the stress state, such as internal stress and / or surface stress, on the part to be subjected to ultrasonic shock treatment of structural products after ultrasonic shock treatment is applied. Is preferred.
[0041]
That is, after applying ultrasonic striking treatment, the surface layer of the part is made into a layered structure, or plastically deformed to make the surface shape difficult to cause stress concentration, and after applying the residual compressive stress, the part and its vicinity In order to suppress environmentally assisted cracking formed by ultrasonic impact treatment when processing that changes the structure of the surface layer, plastic deformation state, stress state, etc., such as plastic processing, correction, heat treatment, welding, etc. As a result, the properties of the surface layer are reduced, and the suppression effect is reduced.
[0042]
Therefore, in the ultrasonic shock treatment method of the present invention, steel For at least a portion of the structural product to be subjected to the treatment, for example, processing such as plastic working, straightening, heat treatment, welding, etc. that changes the surface layer structure, plastic deformation status, stress state, etc. It is preferable to perform the pretreatment before the impact treatment, and it is preferable not to perform such treatment after the ultrasonic impact treatment.
[0043]
In addition to the above-described processes, the pre-processing described above preferably includes a process for inspecting the presence or absence of a crack at a location where environmentally-assisted cracking is a problem and removing the detected crack. That is, visual inspection, penetration inspection, magnetic particle inspection, eddy current inspection, etc. steel By using appropriate means for inspecting cracks in structural products, inspect the presence or absence of cracks at locations where environmentally assisted cracking is a problem, that is, locations where ultrasonic impact treatment is to be performed, and On the other hand, a process for removing this is performed in advance. As a method of removing, an appropriate method such as a method of removing the cracked portion by grinding and cutting with a grinder, a cutting tool or the like, or a method of melt-bonding the cracked portion by welding can be adopted.
[0044]
In particular, when the depth of the removed crack is 3 mm or more, the crack portion is ground and removed, and after overlay welding, the surface of this portion is smoothed by mechanical means such as a grinder or a cutting tool. It is preferable to include a process of finishing the shape and confirming that no crack is detected by the above-described crack inspection process.
[0045]
In the present invention, the above-described pretreatment is performed as necessary, the ultrasonic shock treatment is performed, and then a quality assurance inspection is performed as necessary.
[0046]
In the quality assurance inspection after the ultrasonic impact treatment, the treated surface is plastically deformed to a thickness of 50 μm or more compared to that before the treatment, that is, the surface layer having a thickness of 50 μm or more from the surface has a layered structure. And / or whether the treated surface has a surface shape in which stress concentration hardly occurs.
[0047]
To confirm that the treated surface is plastically deformed to a thickness of 50 μm or more compared to before treatment, make a replica of the treated surface by the sump method and observe the crystal structure or crystal of the treated surface. The grain size can be determined by measuring the crystal grain size by any of ultrasonic particle size measuring devices and judging whether or not 50% or more of the crystal grains are refined compared to the untreated portion. If the crystal grain refinement is less than 50%, the formation of the layered structure is insufficient. Note that the term “miniaturization” used here refers to processing. Absent If the grain size number is 1 or more larger than the crystal grain size of the portion, it is determined that the portion is refined.
[0048]
In addition, in order to confirm whether or not the processing surface has a surface shape that is hard to concentrate stress, for example, a mold is taken using a mold material such as a dental shape material, and the surface shape of the copied replica is inspected. Alternatively, by measuring the displacement of the surface using a highly accurate displacement measuring device such as a laser displacement meter, it is performed by determining whether the treatment surface has a curvature or displacement of the surface on which stress concentration is difficult. be able to.
[0049]
By performing the quality assurance inspection to confirm the surface layer structure or surface shape after the ultrasonic impact treatment by the method as described above, steel It can be confirmed that the environmentally assisted cracking resistance is improved in the part where the environmentally assisted cracking of the structural product becomes a problem.
[0050]
In addition, when the required surface shape or surface structure is not obtained by this quality assurance inspection, it goes without saying that the ultrasonic impact treatment is repeated so that the required surface properties and surface structure are obtained.
[0051]
【Example】
Hereinafter, the present invention will be described by way of examples.
[0052]
A steel (sheet thickness 12 mm) having the composition shown in Table 1 is used as a base material, and a welded portion arc welded using a common metal welding material is used as a test body. steel The structure was a layered structure composed of crystal grains whose major axis was substantially parallel to the surface. As a comparative material, an as-welded specimen was used. In a manner schematically shown in FIG. 4, a welded portion was formed with a bead-on plate at the center of the test piece having a width of 100 mm, a length of 200 mm, and a plate thickness of the original thickness, and was directly used as an environmentally-assisted cracking test piece. Needless to say, a welding residual stress exists in the welded portion of the as-welded specimen.
[0053]
These specimens were washed with 1N Na at 90 ° C. ₂ CO ₃ + 1N NaHCO ₃ It was immersed in an aqueous solution, held at a constant potential (-0.42 V in terms of standard hydrogen electrode standard) using a potentiostat, and tested for 90 days. The test piece after the test was checked for the presence of environmentally assisted cracking by a penetration test and cross-sectional observation.
[0054]
Table 2 shows the ultrasonic shock treatment conditions, and Table 3 shows the results of the crystal grain and environment-assisted cracking test.
[0055]
As is clear from Table 3, the surface layer steel In Examples 1 to 8 of the present invention in which the structure is a layered structure composed of crystal grains whose major axis is substantially parallel to the surface, environmentally-assisted cracking (stress corrosion cracking) does not occur at all. In Comparative Examples 9 to 12 where the treatment is insufficient, environmentally assisted cracking (stress corrosion cracking) occurs, and the effect of the present invention is clear.
[0056]
[Table 1]

[0057]
[Table 2]

[0058]
[Table 3]

[0059]
【The invention's effect】
Book According to the method of the invention, Ultrasonic impact treatment is applied to the part of the steel structure product that causes environmentally assisted cracking, the surface layer becomes a layered structure, and more preferably, the surface has a surface shape that is less likely to cause stress concentration and is subjected to residual compressive stress. Therefore, even if exposed to a corrosive environment, microcracks are unlikely to occur, and even if microcracks are present, progress in the thickness direction of the cracks is suppressed, rupture time is greatly increased, and A steel structure product having excellent resistance can be obtained. Also, By combining the quality assurance inspection after ultrasonic impact treatment, it is possible to confirm that the surface layer of the treated part has a predetermined layered structure and further has a surface shape. Assisted cracking resistance can be improved reliably.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing the progress of stress corrosion cracking cracks, where (a) shows the case where the grain boundary is in a direction perpendicular to the direction of tensile stress, and (b) shows that most of the grain boundary is tensile stress. In the direction parallel to the direction.
[Figure 2] steel It is a figure which shows the structure | tissue before the ultrasonic impact process of a structure product, (a) is a structure | tissue photograph (b), The schematic diagram.
[Fig. 3] steel It is a figure which shows the structure | tissue after the ultrasonic impact process of a structure product, (a) is a structure | tissue photograph (b), The schematic diagram.
FIG. 4 is a diagram showing a sampling state of a stress corrosion test piece.
[Explanation of symbols]
1 ... Base material
2 ... welded part

Claims

The tip of an ultrasonic impact device having a tip hammer with a radius of curvature of 1 to 2 mm at the tip at a location where environmentally assisted cracking is a problem due to concentration or residual stress of the steel structure product, the tip of which has an amplitude of 30 μm to 60 μm Oscillate under any condition of cycle number 40 kHz to 60 kHz, output 0.2-3 kW, amplitude 50 μm to 60 μm, cycle number 25 kHz to less than 40 kHz, output 0.2-3 kW The surface of the steel structure with a thickness of 50 μm or more from the surface is subjected to an ultrasonic impact treatment, and the major axis direction of the crystal grains and the surface are at an angle of ± 10 ° or less, and the major axis length of the crystal grains A method for improving the environment-assisted cracking resistance of a steel structure product, characterized in that a layered structure composed of crystal grains is used in which the ratio of the length to the minor axis direction is 5 or more.

^2. The environment-enhanced cracking resistance improvement of the steel structure product according to claim 1 , wherein the steel where the environment-assisted cracking of the steel structure product becomes a problem is a steel having a tensile strength of 490 N / mm grade ² or more. Method.

Locations environmental assisted cracking of the steel construction product becomes a problem, improved environmental assisted cracking resistance of steel products according to claim 1 or 2, characterized in that it comprises a weld bond portion and / or the weld heat affected zone Method.

Direction and the surface of the long axis of the crystal grains of the lamellar structure is any one of claims 1 to 3, characterized in that the short-axial length of the crystal grains is less angle ± 10 ° and 5μm or less 2. A method for improving the environment-assisted cracking resistance of steel structure products according to item 1.

Wherein prior to application of ultrasonic impact treatment, the steel structure environmental assisted cracking is a part and its vicinity portion becomes a problem of product, according to any one of claims 1 to 4, characterized in that pretreated To improve environmentally-friendly crack resistance of steel structure products.

6. The steel structure product according to claim 5 , wherein the pretreatment is a process of changing an internal stress and / or a surface stress at a location where environmentally assisted cracking of the steel structure product becomes a problem and in the vicinity thereof. To improve environmental resistance crack resistance.

The pretreatment detects a crack of a portion environmental assisted cracking of the metal structure product becomes a problem, the metal structure according to claim 5 or 6, characterized in that it comprises a process of removing the detected crack How to improve environmentally-friendly crack resistance of products.

The ultrasonic impact treatment is further characterized in that the surface shape of the steel structure product where the environmentally assisted cracking is a problem is formed into a shape having a stress concentration factor of 2 or less and compressive residual stress is applied in the vicinity of the surface. The method for improving environment-assisted cracking resistance of steel structure products according to any one of claims 1 to 7 .

The steel structure according to any one of claims 1 to 8, wherein an ultrasonic impact treatment is performed on a portion where environmentally assisted cracking of the steel structure product becomes a problem, and then a quality assurance inspection is further performed. How to improve environmentally-friendly crack resistance of products.

In the quality assurance inspection, the treated surface after the ultrasonic impact treatment has a surface shape in which the thickness of 50 μm or more is plastically deformed, and the treated surface has a stress concentration coefficient of 2 or less. The method for improving the environmentally-assisted cracking resistance of steel structure products according to claim 9 , characterized in that either one or both of them are confirmed.

The plastic deformation in the quality assurance inspection is confirmed by observing the treated surface after ultrasonic impact treatment by a sump method to determine whether 50% or more of the steel crystal grains are finer than the untreated portion. The method for improving the environment-assisted cracking resistance of a steel structure product according to claim 10 , characterized in that:

The plastic deformation of the quality assurance inspection is confirmed by measuring the crystal grain size of the treated surface after ultrasonic impact treatment with an ultrasonic grain size measuring device, and 50% or more of the crystal grains are fine compared to the untreated part. The method for improving the environment-assisted cracking resistance of a steel structure product according to claim 10 , wherein the method is based on judging whether or not the steel structure has been made.

The confirmation of the surface shape that does not easily cause stress concentration in the quality assurance inspection is carried out by using a mold material for the treated surface after ultrasonic impact treatment, and whether the surface obtained by the mold has a surface shape that is difficult to concentrate stress. The method for improving environment-assisted cracking resistance of steel structure products according to claim 10 , characterized in that:

Confirmation of the surface shape where stress concentration hardly occurs in the quality assurance inspection is performed by measuring the treated surface after ultrasonic impact treatment using a displacement meter, and whether the displacement is within the range of displacement of the surface where stress concentration is unlikely to occur. The method for improving environment-assisted cracking resistance of a steel structure product according to claim 10 , wherein the method is to determine whether or not.